Solidification of Highly Undercooled Hypereutectic Ni-Ni3B Alloy Melt

The solidification of undercooled Ni-4.5 wt pct B alloy melt was investigated by using the glass fluxing technique. The alloy melt was undercooled up to ΔT _p ~ 245 K (245 °C), where a mixture of α-Ni dendrite, Ni₃B dendrite, rod eutectic, and precipitates was obtained. If ΔT _p < 175 K ± 10 K (175 °C ± 10 °C), the solidification pathway was found as primary transformation and eutectic transformation (L → Ni₃B and L → Ni/Ni₃B); if ΔT _p ≥ 175 K ± 10 K (175 °C ± 10 °C), the pathway was found as metastable eutectic transformation, metastable phase decomposition, and residual liquid solidification (L → Ni/Ni₂₃B₆, Ni₂₃B₆ → Ni/Ni₃B, and L_r → Ni/Ni₃B). A high-speed video system was adopted to observe the solidification front of each transformation. It showed that for residual liquid solidification, the solidification front velocity is the same magnitude as that for eutectic transformation, but is an order of magnitude larger than for metastable eutectic transformation, which confirms the reaction as L_r → Ni/Ni₃B; it also showed that this velocity decreases with increasing ΔT _r, which can be explained by reduction of the residual liquid fraction and decrease of Ni₂₃B₆ decomposition rate.     

Phase Selection in Undercooled Ni-3.3 Wt Pct B Alloy Melt

The change of eutectic solidification mode in undercooled Ni-3.3 wt pct B melt was studied by fluxing and cyclic superheating. The eutectic structure is mainly controlled by the undercooling for eutectic solidification, ΔT ₂, instead of ΔT ₁, the undercooling for primary solidification. At a small ?T ₂ [e.g., 56 K (56 °C)], the stable eutectic reaction (L → Ni₃B + Ni) occurs and the eutectic morphology consists of lamellar and anomalous eutectic; whereas at a larger ?T ₂ [≥140 K (140 °C)], the metastable eutectic reaction (L → Ni₂₃B₆ + Ni) occurs and the eutectic morphology consists of matrix, network boundary, and two kinds of dot phases. Further analysis declares that the regularly distributed dot phases with larger size come from the metastable eutectic transformation and are identified as α-Ni structure, whereas the irregularly distributed ones with smaller size are a product of the metastable decomposition and tend to have a similar structure to α-Ni as it grows. Calculation of the classical nucleation theory shows that the competitive nucleation between Ni₂₃B₆ and Ni₃B leads to a critical undercooling, ΔT ₂ ^* [125 K < ΔT ₂ ^* < 157 K (125 °C < ?T ₂ ^* < 157 °C)], for the metastable/stable eutectic formation.     

Ni53Mn22Ga25磁性形状记忆合金的相变内耗

研究了Ni53Mn22Ga25磁性记忆合金在190～390K温度范围的内耗行为。结果表明．当合金发生正、逆马氏体相变时出现明显的内耗峰,并伴随模量变化出现极小值。马氏体相变后继续降温。在215K附近出现新的内耗峰,模量明显增高。合金在居里温度发生顺磁及铁磁正反转变时．内耗及模量均不发生变化。对上述现象进行了分析和讨论。     

Microstructure Evolution and Phase Formation of Fe25Ni25CoxMoy Multi-principal-Component Alloys

Metallurgical and Materials Transactions A - Fe25Ni25CoxMoy multi-principal-component alloys (MPCAs) are designed in this study; the ratio of Co and Mo is changed to study the effect on the...     

Pre-bainitic Transformation in Fe-Ni Alloy

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Al-Cu合金亚稳相的价电子结构分析

应用固体电子理论(EET)，对A1-Cu合金时效析出的若干亚稳相的价电子结构进行计算。结果表明，在时效初期，θ”相中Cu原子处于第七杂阶，比金属Cu的原子杂阶低，而最靠近Cu原子的A1原子则处于第五杂阶；由θ”相溶解形成相θ’时，Al原子处于第四杂阶，而Cu原子状态发生较大变化，从基体的第九杂阶上升到第十三杂阶，使得Cu原子的共价电子数有较大幅度的提高，因此形成的θ’亚稳相的最强共价键较θ”亚稳相的要强l倍，这就从价电子结构层次上解释了θ”和θ’亚稳相的热稳定性。     

Metastable Phase Transformation in Ti-5Ta-2Nb Alloy and 304L Austenitic Stainless Steel under Explosive Cladding Conditions

Ti-5Ta-2Nb alloy was clad on 304L austenitic stainless steel (SS) using the explosive cladding process. Both Ti-5Ta-2Nb and 304L austenitic steel were severely deformed due to high pressure (in the gigapascal range) and strain rate (10⁵/s), which are characteristics of explosive loading conditions. Consequent changes produced in the microstructure and crystal structure of both the alloys are studied using electron microscopy techniques. The microstructure of both Ti-Ta-Nb alloy and 304L steel showed evidence for the passage of the shock waves in the form of a high number density of lattice defects such as dislocations and deformation twins. In addition, both the alloys showed signatures of phase transformation under nonequilibrium conditions resulting in metastable transformation products. 304L SS showed martensitic transformation to both ????(bcc) and ??(hcp) phases. Microscopic shear bands, shear band intersections, and twin boundaries were identified as nucleation sites for the formation of strain-induced phases. Ti-Ta-Nb alloy underwent metastable phase transformation to an fcc phase, which could be associated with regions having a specific morphology.     

Al含量对FeV50合金相变的影响及机理研究

对不同Al含量的FeV50进行熔化和随炉冷却试验,用X射线衍射(XRD),扫描电镜(SEM)分析检测物相和微观组织,结果表明:FeV50合金随Al含量的增加,其室温物相由单一的σ相逐渐向α相转变;Al含量存在临界值0.8%,低于0.8%时,FeV50为σ相,超过0.8%,将出现两相共存直至转变为单一的α相。分析认为Al对FeV50合金相变的影响机理表现在两个方面:①随Al含量的增加,合金液的临界晶核尺寸逐渐变大,使得体系中没有过多的能量作为α→σ相变的驱动力;②Al含量越高,其对Fe原子扩散的抑制作用越强,Fe的同素异构转变不能进行,从而阻止α→σ相变的发生。     

Nanocrystal Growth in Thermally Treated Fe75Ni2Si8B13C2 Amorphous Alloy

Thermal treatment of amorphous Fe₇₅Ni₂Si₈B₁₃C₂ alloy leads to crystallization of the stable ??-Fe(Si) and Fe₂B as well as to the metastable Fe₃B phase. The study of the mechanism of crystal growth of the ??-Fe(Si) phase revealed that the mechanism of ??-Fe(Si) growth changes from two dimensional in the early stage to one dimensional in the later stage of crystallization. The Fe₂B phase was found to crystallize through two independent routes: from the amorphous phase and from the metastable Fe₃B phase, which leads to a different mechanism of crystal growth for each route. Both routes exhibit a change in the mechanism of crystal growth: from two dimensional to one dimensional and from three dimensional to two dimensional, respectively. The respective mechanisms of crystal growth correlate well with the observed changes in preferential orientation of the crystallites of the Fe₂B phase.     

The Beta-to-Alpha Phase Transformation in a Uranium Alloy

The behavior of the ß-to-α phase transformation during continuous cooling was studied in uranium containing 359 ppm Si and 233 ppm Fe using dilatometry and metallography. For cooling rates between 0.01 K/s and 125 K/s the transformation temperature was a linear function of the square root of the cooling rate. The average interface migration rate during transformation at 64 K undercooling was 1.04 × 10^?4 m/s. The transformation enthalpy was 2655 J/mole. The data were analyzed using the theoretical model for massive transformation due to Bhattacharya, Perepezko, and Massalski in which the transformation start was controlled by classical nucleation events without a strain energy barrier. The activation energy for atom transport during nucleation was found to be approximately two-thirds that for volume diffusion in uranium.     

α'相形成与铁铬铝合金的脆性

应用电子显微镜、X射线衍射技术,对低温长期使用的铁铬铝电热合金进行分析。结果表明:晶粒长大以及铬原子含量为75%左右的铁铬二元α'相在晶界和基体中析出,是导致合金变脆的主要原因;通常微量碳化物Cr₂₃C₆相以及σ相总是沿晶界分布。     

Some Effects of Parent Phase Aging on the Martensitic Transformation in a Cu- AI- Ni Shape Memory Alloy

Transmission electron microscopy observations have been carried out for a Cu-14 pct Al-4 pct Ni (wt pct) alloy aged in the thin foil state in an electron microscope. It was found that large cuboidal precipitates of theγ ₂ phase and many small domains of a highly ordered phase form in the DO₃ matrix during aging. The small ordered domains form preferentially on matrix antiphase boundaries as well as within the antiphase domains. The formation ofγ ₂ and the highly ordered phase, both of which are rich in alloy content, depletes the matrix of solute and thus raises the transformation temperaturesM _s andM _f. The small domains of the highly ordered phase prevent the propagation and reversion of martensite plates, leading to higherM _s-M_f andA _fins-A_f temperature intervals.     

CuZnAl合金中塑性形变诱发的相变

通过一个扭转循环使CuZnAl合金产生塑性形变,用电阻法对塑性形变诱发的相变以及塑性形变的减小对诱发形成相的影响进行研究。结果表明:减小样品的塑性形变使已形成的应力马氏体消失,也使再取向马氏体部分消失。此外,在相变温区,塑性形变将使未完全长大的热弹性马氏体长大,而塑性形变的减小对长大后的热弹性马氏体没有影响。     

Strain-Induced Phase Transformation and Nanocrystallization of 301 Metastable Stainless Steel Upon Ultrasonic Shot Peening

The following study investigated the strain-induced phase transformation in metastable austenitic 301 stainless steels via an ultrasonic shot peening treatment (USP) for 5 to 30 minutes. Following the USP, the microhardness increased to a depth of 400 μm and from 200 to 400 HV. The deformed grains and the phase transformation were monitored via X-ray diffraction and electron backscattered diffraction analysis. The grain evolution was studied via transmission electron microscopy. Approximately 500 nm α′-martensite grains formed in the top-most region after 5 minutes of the USP treatment. The grains were then further refined to ~?100 nm when the peening time increased to 10 and 15 minutes. The grains refined down to tens of nanometers after the specimen was treated for 30 minutes, where the phases were composed of α′-martensite (~?50 nm). There was a mixture of austenite with α′-martensite (~?25 nm). The grain refinement and the phase transformation of austenite to α′-martensite during ultrasonic shot peening were systematically investigated.     

再结晶对Ni47Ti44Nb9形状记忆合金棒相变和织构的影响

利用光学显微镜（OM）,DSC和XRD,研究了直径19 mm Ni47Ti44Nb9热轧棒材再结晶对相变和织构的影响,为该合金棒的实际应用提供理论依据。结果表明：热轧棒从450℃开始发生明显再结晶;从850℃/90 min真空退火棒材的中心到边缘,再结晶晶粒尺寸减小,Ms降低,马氏体转变区间变窄,热滞增大;热轧棒中形成了较强的γ丝织构,600℃/90 min退火后,γ丝织构强度稍有下降,退火温度升到850℃,丝织构明显减弱,等密度线发生漫散,形成了新的织构组分,这有利于改善棒材径向方向的恢复应变。     

Mn-Al-C永磁合金相变动力学及τ相的稳定性

本文研究了成分为Mn 69.40%、Al 29.86%、C0.58%3元合金的相变动力学和3种不同成分Mn-Al-C合金的恒温转变过程,还用示差热分析法测定了合金在加热过程中相变的临界温度。实验结果表明:合金的恒温转变过程随成分不同而变化,处于亚共析成分时,τ相分解先析出γ相;处于过共析成分时,τ相分解先析出β-Mn相。τ相的稳定性取决于Mn/C比,含碳量一定,随Mn含量增加,τ相稳定性降低。必须使合金中的含碳量超过其在τ相内的溶解度极限,才能提高τ相的稳定性。